Waveguide structure for linear particle accelerators having an undulating configuration



July 26, I966 G. AZAM ETAL 3,263,116

WAVEGUIDE STRUCTURE FOR LINEAR PARTICLE ACCELERATORS HAVING AN UNDULATING CONFIGURATION F1191 March 13, 1962 2 Sheets-Sheet 1 DIAMETERS u l 0 1 Fig.1

INVENTORSI GUY 424M HUEZRT 16300767 ATTOR EY July 26, 1966 e. AZAM ETAL 3,263,116

WAVEGUIDE STRUCTURE FOR LINEAR PARTICLE ACCELERATORS HAVING AN UNDULATING CONFIGURATION Filed March 13, 1962 2 Sheds-Sheet 2 5 7 (auncuso BEAM) In .55" LL.

United States Patent 6 Claims. (a. 3153.6)

The present invention relates to a novel wave guide structure, and more particularly to a novel wave guide structure for linear accelerators which effectively prevents the propagation of certain unwanted modes.

The linear accelerators which operate by interaction with a traveling wave utilize a wave guide constituted by a geometrically periodic structure, generally by -a cylinder of revolution containing transverse irises distributed at predetermined intervals, which are fixed to the cylinder along the periphery thereof and pierced at their center by a hole enabling passage of the beam of accelerated particles and coupling therebetween the cavities defined between the successive irises.

This structure, as all periodic structures, may transmit waves at different modes each of which possesses its own bandpass. The mode chosen for the acceleration of the particles is that which, when propagating through the cavities coupled with one another by holes having a diameter which diminishes progressively or in a stepwise manner, is derived by continuity from the mode TM of the cylindrical cavity defined by the space comprised between two irises 'in which there would be no coupling holes or apertures.

The other possible modes are parasitic modes which can, under certain circumstances, disturb functioning of the normal mode by removing energy therefrom. This trouble becomes evident, in general, with high currents of the accelerated beam, and particularly by the fact that the pulse of the beam collected on a target placed at the end of the accelerator presents deformations consisting either in a simple shortening in time with respect to the starting pulse or in a clip or saddle in the top of the pulse, of location, depth and width which vary with the respective cases.

The present invention aims at a periodic accelerator guide structure which avoids the above-indicated drawbacks.

Applicants studies and research have resulted in a determination that the removal of energy from the normal mode in favor of parasitic modes takes place through the harmonics which are naturally present in the beam current. It is known, in fact, that this current is rich in harmonics of the frequency of the wave utilized. If, therefore, the guide or portions thereof allow the propagation of at least one higher mode, the band of which contains the frequency of the harmonic of the given order, then the beam induces in the guide, by means of the harmonic in question, a wave whose structure corresponds to the mode mentioned and the energy of which, passed within that wave at the expense of the kinetic energy of the beam, will be dissipated within the walls of the guide. The result thereof is a drop or lowering of the efliciency of the accelerator and a limitation of the performances thereof, particularly a limitation of accelerated current taking into account the desire to avoid the above-mentioned deformations.

It has also been established that the aforementioned removal of energy occurs in a preponderant manner through the second harmonic of current of the beam (harmonic at the frequency 2 the fundamental fre- "ice quency of the propagated wave being 1) and for the benefit of the mode of propagation TM The present invention provides means for preventing the tota lor partial coincidence, along the path of propagation, between the band-pass of the mode TM and the band of the second harmonics of the frequencies contained in the bandpass of the normal mode utilized for the interaction with a view toward the acceleration of the beam.

It has been found that the measures and dispositions taken in this connection with respect to the mode TM are equally valid with respect to the other possible parasitic modes, i.e., that the removal of energy for the benefit of these modes is prevented or considerably attenuated simultaneously with the prevention of the propagation of the mode TM within the band of the second harmonics of the band of the normal mode.

The means according to the present invention consist in substituting for the usual law of variation of the diameter of the coupling hole along the section of the accelerator, i.e., a law of progressive or step-wise decrease between a maximum entrance diameter and a minimum exit diameter, an undulating law between these two diameters, the undulations being either periodic or non periodic and extending at least over a length of guide over which, with a design according to the known art, this diameter would exceed a certain threshold which only depends on the frequency used and corresponds to the beginning of overlap between the band of the mode TM and the band of the second harmonics of the band of the normal mode. Applicants have been able to establish, in fact, that this partial or total overlap is produced exclusively in the parts of the guide where the diameter of the coupling hole exceeds the aforementioned threshold. It is supposed that to design a section of the accelerator according to the known art in accordance with the performances desired and utilizing a frequency of a given value would lead to an inlet or entrance diameter which is larger than the aforementioned threshold, and to an outlet or exit diameter which re smaller than the same threshold. The variations of the diameter of the coupling hole in the guide according to the present invention will preferably be progressive to avoid the unwanted reflections of the wave, whereas the phase velocity which must be locally adjusted to the speed of the electrons along the guide will be brought to the desired value by acting upon one or several other dimensions of the cavity, as is known in the art.

Accordingly, it is an object of the present invention to provide a wave guiding structure for linear accelerators which eliminates in a very effective manner and by simple means the shortcomings and drawbacks encountered with the prior art constructions.

It is another object of the present invention to provide a wave guiding structure for linear accelerators which effectively prevents any interference with the proper operation of the accelerator in the normal mode by parasitic modes.

A further object of the present invention resides in the prov1s1on of a wave guiding structure for .a linear accelerator which may be operated with high electron beam currents without endangering the efficiency by interference from parasitic modes removing useful energy from the electron beam to the detriment of the usable output.

It is a still further object of the present invention to provide a wave guiding structure for linear accelerators which assures an output pulse representing a reproduction substantially corresponding in duration, general shape and contour to the input pulse.

It is a further object of the instant invention to provide .3) a wave-guiding structure for linear accelerators which will transmit the mode TM of the travelling wave and at the same time transmit all parasitic modes especially the mode TM which may be excited by the second harmonicof the fundamental excitation frequency of the IIIOdB TMg o.

Still another object of the present invention resides in the provision of a wave guiding structure for linear accelerators achieving all of the foregoing aims and objects without changing the performance thereof from the point of view of energy level as compared to the prior art structures.

These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, one embodiment in acordance with the present invention, and wherein FIGURE 1 is a diagram of curves given only by way of explanation;

FIGURE 2 is an axial cross sectional view through a prior art guide construction, and

FIGURE 3 is an axial cross sectional view through a guide in accordance with the present invention to be substituted for the guide of FIGURE 2.

FIGURE 1 is a diagram showing, along the abscissa, the iris diameters d, and, along the ordinates, two scales, the one on the left corresponding to the frequencies f at an arbitrary scale, and the other, on the right, being double of the former and corresponding to the frequencies 2f at the same scale. Along these ordinates has been represented the variation of the band-pass of the desired normal mode derived from the mode TM the frequencies of this mode being read on the left scale and the band-pass being contained between the curves 1 and 2. On the other hand, there has been represented the variation of the band-pass of mode TM the frequencies of this mode being read on the scale at the right and the band-pass of this mode being contained between curves 3 and 4. All of these curves have been found experimentally with a particular device, but may be considered as typical for the guide structure of cylindrical form loaded with irises, independently of its dimension.

As is well known, the modes which may be generated in a wave guide are distinguished from each other by the frequency bands susceptible of exciting them respectively, the position of the band depending in general on the structure of the circuit, for a given structure of its design, and for a given design of the circular iris structure, of the diameter of the holes in the irises. This dependence is represented in FIGURE 1. It is evident therefrom that, if the diameter of the hole in the iris is D it is necessary that the excitation frequency be comprised between ma and mb (as read on the left-hand scale), if the mode TM is to be excited, and that, for exciting the mode TM the frequency must be comprised between me and me (as read on the right-hand scale). If the diameter of the hole is D and if the mode TM is to be excited, the frequency must be comprised between ng and uh (on the scale at left) and, for exciting the mode TM it is necessary that the frequency be between ni and nk as read on the right-hand scale.

It may be seen that, when the diameter has exceeded the value d corresponding to the point of intersection A of curves 2 and 3, the band of the second harmonics of the frequencies of the band-pass of the normal mode begins to overlap with the band-pass of the mode TM so that the unwanted phenomena mentioned above begin to appear when a certain threshold of the diameter has been exceeded, which is variable with the frequency of the chosen wave. For example, for a chosen frequency f the threshold is given by the diameter d corresponding to point B of the ordinate f on curve 3. This threshold can thus be determined experimentally for each given structure and for each frequency of the selected wave.

FIGURE 2 is a cross section through a guide portion calculated to obtain certain desired performances and made in accordance with the prior art. It is supposed that this realization has ended in the constitution of the guide section by a certain number of cavities of which the diameter of the coupling holes varies between a maximum value d at the inlet and a minimum value d at the outlet. For example, this variation has been realized in three steps or gradients the guide being constituted by the section I having cavities 5 with holes of d by the section III having cavities 6 with holes of intermediate diameters, and by section V having cavities 7 with holes of d These sections are, respectively, connected with one another by sections II and IV in which the diameter of the coupling holes varies progressively. The inner diameters of the cavities are adjusted to maintain the appropriate law of variation of the phase velocity of the high frequency wave propagating through each cavity of the guide. It will also be supposed that the examination of the modes of propagation within the guide has shown the critical threshold d mentioned hereinabove in connection with FIGURE 1 to be smaller than a but larger than d i.e., that the unwanted phenomena mentioned hereinabove can be produced in the first part of the guide L, up to the point where the diameter of the coupling hole becomes smaller than d Acording to the present invention, after having calculated the guide for the desired performances, and after having determined in particular the number of cavities necessary for realizing the known structure of FIGURE 2, such structure is realized according to FIGURE 3, i.e., by substituting for the decreasing law of the diameter of holes of FIGURE 2, an undulating law, along the same section, between values d and d The guide being generally constituted by a group of individual cavities, it is sufficient to disassemble or take apart the guide of FIGURE 2 and to regroup the cavities by intercalating a certain number of cavities 7, for example, periodically between a certain number of cavities 5, the intermediate cavities of sections II, III and IV being utilized for matching together the successive diameters, thereby avoiding too abrupt a transition. The becomes thus composed of a certain number of sections 1 to V, of which each has a length smaller than L, and of which each comprises cavities with holes having a diameter a' which cut off the propagation of the unwanted modes through the cavities having holes of a diameter larger than al If all of the cavities of FIGURE 2 are utilized to form the new structure of FIGURE 3, the average diameter of the coupling holes along the section remains unchanged, which means that the performance of the guide from the standpoint of energy level has not been changed.

By way of example, for an accelerator functioning in the 3,000 mc. band, cavities may be chosen which vary between a diameter of 83 mm. with a hole of d =27 mm., and a diameter of mm. with a hole of d 18 mm. The pitch of the cavities would be of 25 mm. The length L covered by 15 cavities of sections I and II of FIGURE 2 is, in this case, equal to 375 mm. The entirety of sections I to V being realized with 40 cavities, their distribution between the sections I to V' of FIGURE 3 produces 8 cavities per section, i.e., a section of 200 mm. length. This length is well below the value L defined hereinabove.

The drawing also shows a series of particle bunches 8 in FIGURE 3, resulting from bunching of the beam by interaction with the traveling wave propagating through the cavities 5 and 7.

Theory and experience both have shown that the energy removed from the normal mode in favor of the mode TM through the second harmonic of the frequency of the wave utilized in the current of the beam is subjected, between two consecutive sections such as I defined between two successive groups of cavities 5, to a phase variation the result of which is the extinction of the parasitic mode at the end of a certain path. The described structure is thus effective to eliminate the undesirable phenomena mentioned hereinabove.

While we have shown and described one embodiment in accordance with the present invention, it is understood that the same is not limited thereto, but is susceptible of many changes and modifications within the spirit and scope thereof, as known to a person skilled in the art, and we therefore do not Wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications thereof as are encompassed by the scope of the appended claims.

We claim:

1. A linear particle accelerator comprising a wave guide having a geometrically periodical structure, comprising transverse irises periodically distributed along said guide and having in their center coupling apertures between cavities defined between two successive irises, the size of said apertures varying substantially according to an undulatory law along said guide between a minimum size lower than the cut-out threshold of the TM mode, and a maximum size higher than the said threshold and a particle beam moving within said guide in inter-action with the traveling wave propagating therein.

2. A linear particle accelerator comprising a wave guide having a geometrically periodical structure, comprising transverse irises periodically distributed along said guide and having in their center coupling apertures between cavities defined between two successive irises, the size of said apertures varying substantially according to an undulatory law along said guide between a minimum size lower than the cut-out threshold of the TM mode, and a maximum size higher than the said threshold, a particle beam moving within said guide in inter-action with the travelling wave propagating therein, and the sizes of said cavities being variable in dependence on the sizes of corresponding central apertures, to adjust the phase velocity of the travelling wave propagating within the guide to the velocity of th accelerated particle.

3. In a wave guiding structure for linear particle accelerators having a substantially geometrically periodical structure eifectively formed by substantially transverse irises periodically distributed along said guide and provided in the centers thereof with coupling apertures between cavities, the improvement essentially consisting in means in said wave guiding structure preventing the propagation therein of waves of predetermined frequencies that induce parasitic modes of operation, said means being effectively constituted by varying the size of said apertures along said guide substantially in accordance with an undulatory law between a minimum size lower than the cut-out threshold of the TM mode, and a maximum size higher than th said threshold and a particle beam moving within said guide in inter-action with the travelling wave propogating therein.

4. A wave guiding structure for a linear particle accelerator comprising a particle beam moving within said guide in inter-action with the travelling wave propagating therein, first means constituting a substantially geometrically periodical structure eifectively forming a plurality of cavities provided with coupling means therebetween in the form of .central apertures to enable passage therethrough of the accelerated particles, said first means effectively providing a predetermined band-pass for the fundamental frequencies and harmonics thereof of waves, the structures of which correspond to the desired modes, said central apertures forming an undulating pattern along the length of said wave guiding structure effectively preventing propagation therealong of waves the structures of which correspond to parasitic modes of which the frequencies are contained in said band-pass along a length of the guide equal to or greater than a wave length of said waves.

5. In a wave guiding structure for linear particle accelerators having a given band-pass of frequencies and utilizing a mode neighboring the TM mode for the useful mode of operation and a particle beam moving within said guide in inter-action with the travelling wave propagating therein, the improvement essentially consisting in means in said wave guiding structure for elfectively preventing an overlap along the path of propagation for a distance equal to or greater than a Wave length between the bandpass of the TM mode and the band of the second harmonics of the frequencies contained within the band-pass of the operating mode.

6. In a Wave guiding structure for linear particle accelerators having a substantially geometrically periodical cavity structure effectively formed by plural cavity means including substantially transverse irises periodically distributed along said guide and provided in the centers thereof with coupling apertures between cavity means, said cavity means in said wave guiding structure preventing the propagation therein of waves of predetermined frequencies that induce parasitic modes of operation, said cavity means being effectively constituted by varying the size of said apertures along said guide substantially in accordance with an undulatory law between a minimum size lower than the cut-out threshold of the TM mode, and a maximum size higher than the said threshold and a particle beam moving Within said guide in inter-action with the traveling wave propagating therein.

References Cited by the Examiner UNITED STATES PATENTS 2,623,121 12/1952 Loveridg 33331 2,641,731 6/1953 Lines 333 FOREIGN PATENTS 969,886 5/1950 France. 655,410 7/1951 Great Britain. 278,418 10/ 1 Switzerland.

OTHER REFERENCES Harvey: Microwave Engineering, New York, Academic Press, 1963, page 1005 relied on.

ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, Examiner. L. ALLAHUT, Assistant Examiner. 

2. A LINEAR PARTICLE ACCELERATOR COMPRISING A WAVE GUIDE HAVING A GEOMETRICALLY PERIODICAL STRUCTURE, COMPRISING TRANSVERSE IRISES PERIODICALLY DISTRIBUTED ALONG SAID GUIDE AND HAVING IN THEIR CENTER COUPLING APERTURES BETWEEN CAVITIES DEFINED BETWEEN TWO SUCCESSIVE IRISES, THE SIZE OF SAID APERTURES VARYING SUBSTANTIALLY ACCORDING TO AN UNDULATORY LAW ALONG SAID GUIDE BETWEEN A MINIMUM SIZE LOWER THAN THE CUT-OUT THRESHOLD OF THE TM210 MODE, AND A MAXIMUM SIZE HIGHER THAN THE SAID THRESHOLD, A PARTICLE BEAM MOVING WITHIN SAID GUIDE IN INTER-ACTION WITH THE TRAVELLING WAVE PROPAGATING THEREIN, AND THE SIZES OF SAID CAVITIES BEING VARIABLE IN DEPENDENCE ON THE SIZES OF CORRESPONDING CENTRAL APERTURES, TOP ADJUST THE PHASE VELOCITY OF THE TRAVELLING WAVE PROPAGATING WITHIN THE GUIDE TO THE VELOCITY OF THE ACCELERATED PARTICLE. 